VAXmate PSU

[Mattis Lind]

onsdag 8 april 2020 skrev Rob Jarratt <robert.jarratt at ntlworld.com>:

> I will look at all the suggestions, particularly of a failure on the
> secondary side. Something must have burned up, because there was a distinct
> burning smell after the initial failure, although I have never been able to
> see any physical damage to anything, despite looking many times.
>
>
Aha. Don’t think I seen you writing about that before, or did you? It might
be very hard to find the source some times. Even just a small burn will
give quite some smell. Check ALL semiconductors very carefully.

>
>
> But the thing that really puzzles me is that, after correcting the probes
> to include the D19 anode, there doesn’t seem to be anything that would
> cause D19 to trigger. Am I reading the trace wrong?
>
>
It is very hard to tell from the traces what is going on since the
resolution is too low.  Use a faster timebase. 5 or 10 microseconds. Find
out if you can trigger on something that happen only when it stops. Like
channel 2 negative slope.

/Mattis

>
>
> Thanks
>
>
>
> Rob
>
>
>
> *From:* Mattis Lind <mattislind at gmail.com>
> *Sent:* 08 April 2020 07:42
> *To:* rob at jarratt.me.uk; Rob Jarratt <robert.jarratt at ntlworld.com>;
> General Discussion: On-Topic and Off-Topic Posts <cctalk at classiccmp.org>
> *Subject:* Re: VAXmate PSU
>
>
>
>
>
>
>
> Den ons 8 apr. 2020 kl 00:34 skrev Rob Jarratt via cctalk <
> cctalk at classiccmp.org>:
>
>
>
> > -----Original Message-----
> > From: cctalk <cctalk-bounces at classiccmp.org> On Behalf Of Brent Hilpert
> via
> > cctalk
> > Sent: 06 April 2020 21:07
> > To: General Discussion: On-Topic and Off-Topic Posts
> <cctalk at classiccmp.org>
> > Subject: Re: VAXmate PSU
> >
> > On 2020-Apr-05, at 11:12 PM, Rob Jarratt wrote:
> > >>
> > >>> I have obtained a scope trace as you suggest. R32 is still lifted so
> > >>> the
> > >>> UC3842 is powered by the bench PSU, but I am using the full 240VAC
> > >>> (no variac). The channels are:
> > >>> 1.        Ch1. 555 timer.
> > >>> 2.        Ch2. D19 Anode
> > >>> 3.        Ch3. D19 Gate.
> > >>> 4.        Ch4. Q1 Source.
> > >
> > > Sorry, that looks like a cut and paste error, here is the link to the
> > > scope picture
> > > https://rjarratt.files.wordpress.com/2020/04/h7270-primary-scr-trigger
> > > .png
> > >
> > > I used a 100ms timebase for the capture and then "zoomed in" a bit
> >
> >
> > You would need to zoom in far more to see what's going on when the SCR
> > triggers, to cover just a few cycles around the trigger time.
> >
> > Once an SCR has been triggerred, the gate becomes a voltage/current
> supply, a
> > diode drop above 0.
> > You see this on your trace in that after triggerring the gate sits at
> something +V
> > above 0.
> > The spike you see may just be an artifact of the internal SCR trigger
> action.
> > I presume you see some increased current draw from your bench supply for
> the
> > 3842 after the SCR triggers.
> >
> > What's up with channel 2? Above you say it's D19 anode which is 3842 Vcc
> but
> > it shows on the trace as just noise around 0V.
> >
> > I would still suggest that you scope the state of the secondary-side
> crowbar -
> > the gate of Q2, and base of Q4.
> > Should be simple to do, before trying to remove or disconnect the main
> > transformer.
>
> Oh dear! After Brent's question about D19 anode, I realise that the probe
> was connected to the cathode! I have now done it again with the probe
> connected to the anode. I have taken two images of the same capture, one at
> low resolution to show the overall behaviour
>
> https://rjarratt.files.wordpress.com/2020/04/primary-side-shutdown-1.png
>
> And one zoomed in to show what happens when the SCR shuts down.
>
> https://rjarratt.files.wordpress.com/2020/04/primary-side-
> shutdown-detail-2.
> png
>
> The channels are the same as before, namely:
> Ch1. 555 timer.
> Ch2. D19 Anode (now corrected as it was previously the cathode!)
> Ch3. D19 Gate.
> Ch4. Q1 Source.
>
> I got an earlier trace which showed the D19 anode at 9V, which is under the
> Undervoltage Lockout threshold, but I have not been able to repeat it.
>
> I don't fully understand the debate about using the variac.
>
>
>
> I am not going to debate this either since I know what I have been doing
> for years and it works perfectly well for me. I have fixed the bigger PSUs
> in a VAX 11/750 (one broken switch transistor and multiple broken output
> rectifiers). PSU in NORD-10/S (most carbon composition resistors had gone
> out of spec). PSUs in many smaller machines as well.
>
>
>
> I prefer to work in circuits where I can fiddle around without the danger
> of getting killed all the time. Regardless of use of HV differential probe
> it can be dangerous. Running it on 50VAC with a protection transformer do
> expose a lot of problems already and you can poke around safely in the PSU.
>
> I have not yet seen a problem that wasn't seen at low voltage, but I
> expect there could be semiconductors that experience breakdown that occur
> at lower than specified voltage.
>
>
>
>
>
> However, my
> measurements appear to suggest that when I use the variac the SCR triggers
> because of what appears to be a genuine overcurrent detected by R13. I
> think
> this is because the duty cycle at low AC input voltages is 50% (rather than
> about 10% or less as per the trace I have just taken), and I measured 2V
> across R13, which does seem to be enough to trigger the SCR. When I use
> 220VAC, the voltage across R13 does rise to 6V, which should also trigger
> the SCR I think, except that the peak last a lot less and so perhaps the
> fact that the 6V last for a brief period is insufficient to trigger it?
>
>
>
>
>
> On the issue of duty cycle. If we look at this from the start up
> perspective rather than the steady state perspective. At startup there are
> no stored energy in the output filter capacitors. The voltage on the output
> is thus 0. As soon as the PSU is doing its first switching pulse energy is
> transfered as the main switch transistor is cutting off. The energy is
> transfered into the capacitor and into the load. The voltage is starting to
> increase.
>
>
>
> The duty cycle generated by the PWM circuitry is in pure relation to the
> voltage error, i.e. the difference between output voltage and reference
> voltage. In essence it is a P-regulator.
>
>
>
> When there are 0 Volt out the duty cycle will be at the maximum. Nothing
> strange about that. But what is maximum duty cycle? It depends on the
> circuitry used. The UC3842 can do up to almost 100% duty cycle. However it
> may be wise to limit duty cycle in a flyback design so that the transformer
> is not saturated. I am not sure if there is some kind of duty cycle
> limitation in this circuit though.
>
>
>
> So if it can handle 50% duty cycle at startup it should be able to handle
> it at any time. Besides it would be incredible weird to design a circuit to
> use a 10% duty cycle at its standard operating point and detecting over
> current at 50%. Then you have much less head room for load and input
> variations.
>
>
>
> I am more or less convinced that what you see on the primary side is a
> result of some kind of fault on the secondary side.
>
> A very common problem is short-circuit rectifier diodes on the secondary
> side (D12, D11, D21, D22, D23, D24). They can be difficult to measure
> correctly in circuit since the resistance of the secondaries of the
> transformer is so low. Depending on type you can either desolder them
> completely or just lift one end of them.
>
>
>
> My experience is that electrolytic capacitors seldom short circuit. They
> probably boil and explode instead. Tantalum capacitors often short circuit.
> Some of them goes into fire other just stay short circuit. So check for
> tantalum capacitors and try to measure them for short circuit.
>
>
>
> You have a crowbar on the secondary side. Are you sure that one hasn't
> triggered? If you still run on variac you can disable the crowbar circuit
> by removing the SCR and ramp up the voltage slowly to see if that makes any
> difference.
>
>
>
> Breaking the feed back loop:
>
>
>
> R23 seems to be in the feedback path. If you lift it and insert a voltage
> from a lab supply here you could simulate the output voltage and study the
> behaviour of the UC3842 for different feed back voltages. You will see that
> it will stay on max duty cycle up until close to the nominal voltage and
> over a very small span change to almost no pulse out at all. This is due to
> the gain of the circuit.
>
>
>
> /Mattis
>
>
>
>
> I have seen the suggestions to study the waveforms at a much higher
> resolution. What I am doing is setting the overall timebase in the 100ms
> range so that I can trigger on when the 555 starts to oscillate and capture
> the whole period of operation until the SCR triggers. I can then zoom in,
> as
> can be seen from the trace provided in this email. I hope that is good
> enough, or am I missing some problem with doing it this way?
>
> I would like to follow Mattis's suggestions (and other people have said it
> too) to break the feedback loop, but it does look difficult to know how
> best
> to do it.
>
> I also understand Brent's suggestion that the gate spike is just the result
> of the SCR triggering, rather then the cause of the trigger. I had wondered
> if that might be the case.
>
> Regards
>
> Rob
>
>